Glue detector



July 10, 1962 F. MALTBY 3,043,993

GLUE DETECTOR Filed Dec. 24, 1959 FIG. 2

LEVEL OF OSCILLATION //X PLATE CURRENT INVEN TOR. FREDERICK L. MALTBYCONDUCTIVITY LEVEL OF OSCILLATION United States Patent 'Ofiil 3,043,993Patented July 10, 1962 Frederick L. Maltby, 1417 Edge Hill Road,Abington, Pa.

Filed Dec. 24, 1959, Ser. No. 861,932 6 Claims. (Cl. 317--146) Thepresent invention generally relates to an apparatus for detecting thepresence of glue or other conductive adhesives on paper or likematerials. More specifically, the present invention is concerned with asystem in which the presence or absence of glue on an article affectsthe coupling between a plurality of sensing electrodes.

In the manufacture of paper products such as bags, glue is generallyapplied to the paper as it comes from a supply roll prior to the cuttingand folding steps of manufact-ure. As a result, unless inspection isprovided, hundreds of yards of paper can pass through the gluingstation, be cut and folded prior to the discovery of a malfunction inthe gluing operation. This can result in an enormous waste of rawmaterial, time, and labor. A similar problem exists in the paper boxindustry, where malfunctioning of a gluing operation can result in thefilling of unglued boxes and the like, with the loss of both contentsand container. Today, even after a half-century of automation in theproduction of paper products, it is standard practice to provide aninspector at each machine to insure proper gluing. In specific cases,however, production rates are such that even manual inspection isinadequate to insure proper gluing of the product.

Accordingly, it is anobject of the present invention to provide aninexpensive, reliable, automatic means to detect the presence or absenceof glue on paper or the like which can :be added to existing machineswithout modification thereof.

One characteristic of substantially all of the commercially used gluesand pastes is a conductivity which is, in general, several orders ofmagnitude higher than that of the paper products to which the glue isapplied. Accordingly, this characteristic can be utilized in detectingthe presence of glue on such products. It is obvious, however, that itis impractical to accomplish a conductivity measurement by making directcontact with the glued surface.

It is therefore, another object of the present invention to providemeans for utilizing the conductivity of glue to detect its presence orabsence from the material without directly contacting the glued paper.

Still another object of the present invention is to provide a new andimproved sensing element adapted to provide capacitive couplings to theglued surface in such a manner that the presence or absence of glueprovides a usable impedance change.

It is, accordingly, a further object of the present invention to providea glue-detecting apparatus in which the sensing elements may bepositioned on the reverse side of the material from which the glue isapplied thereby avoiding contamination of the sensing element with glue.

A still further object of the present invention is to provide a gluedetecting apparatus which is insensitive to the moisture content of thematerial to which the glue is applied.

In an apparatus designed to be sensitive to moisture, as typified bythat disclosed by Wilfred H. Howe, in U. S. Patent 2,718,620, a capacitychange caused by the so-called fringe effect, that is, the electro-saticfield that bulges out from the edge of condenser plates at the gapbetween the plates and which penetrates material placed adjacent to thesurface of the plates, is utilized to effect a moisture measurement. Inglue detection, however, the effects of the fringe field is to introduceerrors, and hence this effect must be minimized. This is because anyfringe field present will penetrate the material on which the glue isapplied, not the glue, since it is desirable to have the materialbetween the sensing electrodes and the glue. In addition, glue is oftenapplied to material which has a high or varying moisture content. Thus,in detecting glue, it is necessary to utilize a sensing element whichwill provide coupling with the glue through the material to which theglue is applied without any substantial fringe field effect.

Since the fringe field occurs primarily at the gap hetween the capacitorelectrodes it is desirable in apparatus depending upon the effect toutilize a plurality of thin, closely spaced electrodes to increase thesensitivity of the apparatus, with the sensitivity increase achievedbeing proportional to the increase in the number of capacitor plates. Inthe present invention, the effect of the fringe field is minimized byutilizing relatively few electrodes having dimensions which are largecompared to their spacing. In addition, an insulating spacing member isemployed between the electrodes and the material to which the glue isapplied to minimize the fringe field penetrating the material. As aresult of this construction, the system is made insensitive to thecharacteristics of the material to which the glue is applied.

It is also an object of the present invention to provide a sensingelement for the detection of glue having a wipable insulating filmadapted to protect the sensing electrodes from wear and contamination.

' in accordance with the present invention, there is provided a sensingelement adapted to be positioned adjacent to a moving web of paper orthe like on the reverse side of said paper from which glue is appliedand opposite to the applied glue. The sensing element comprises one ormore of energized electrodes spaced from each other by groundedelectrodes and arranged in an in-line relationship, each of the saidelectrodes being adapted to provide capacitive coupling with the glueapplied to the paper. The sensing element has a wipable insulating filmadapted to protect the sensing electrodes from abrasion andcontamination and minimizing fringe field effects. The sensing elementis connected to control the tuning of the plate circuit of a tune-plateand an tune-grid oscillator. In this manner, the presence or absence ofglue on an article positioned adjacent to the sensing element controlsthe oscillation or non-oscillation of the oscillator. A relay isprovided in the plate circuit of the oscillator which may be adapted togive an indication of or perform a control function in accordance withthe presence or absence of glue on the article tested.

A better understanding of the present invention may be had from thefollowing description when read with reference to the accompanyingdrawings of which:

FIG. 1 is a schematic wiring diagram of the glue detector of the presentinvention;

FIG. 2 is a graph showing the level of oscillation and plate currentplotted against glue conductivity for the apparatus shown in FIG. 1; and

FIG. 3 is a perspective view of the capacitor sensing element of thepresent invention.

Referring now to FIG. 1, there is shown an embodiment of the presentinvention adapted to detect the presence or absence of a layer of glue 1upon a moving strip of paper 2. As shown schematically, the paper 2moves in the direction of the arrow, as from a supply roll, past a gluewheel 3 which applies a layer of glue thereto from a hopper 4. Afterbeing glued, the strip of paper 2 moves past a sensing element 5, whichis positioned adjacent to the path of paper travel on the side of thepaper opposite to which the glue is applied. The sensing element 5 isconnected by means of a shielded cable 6 to an oscillator 7 to controlthe oscillation or non-oscilla- 3 tion of the oscillator 7 in accordancewith the presence or absence of glue on the strip of paper 2.

The oscillator 7 is a tuned-plate, tuned-grid crystal controlledoscillator utilizing the triode section 8 of a dual triode vacuum tube 9as its amplifying element. Direct current energization for the triode 8is supplied by means of the other triode section 11 of the duel trode 9,operated as a diode with the grid directly connected to the plate. Theplate of the tn'ode 11 is connected through the secondary winding 12 ofa transformer 13 to ground. The transformer 13 has a primary winding 14adapted to be connected to a suitable source of alternating current, notshown. The cathode of the triode 11 is connected to the plate of thetriode 8 by means of a relay coil 15 and a variable inductor 16. Therelay coil 15 has associated therewith contacts 17, which may beutilized to control the energization of an alarm, a recorder, or otherform of exhibiting device to indicate the presence or absence of glue onthe paper 2. The

grid circuit of the oscillator 7 includes a crystal 18 and a resistor19, both connected between the grid of the triode 8 and ground. The gridof the triode 8 is also connected to the cathode of the triode 11 bymeans of a resistor 21 and the relay coil 15. As will be explained inmore detail hereinafter, the resistor 21 has substantially no effect onthe operation of the circuit when it is in oscillation, but serves toincrease the plate current flowing when the circuit is not oscillating.The tunedplate circuit of the oscillator 7 comprises the sensing element5, connected between ground and the plate of the triode 8 by means ofthe shielded cable 6 and the variable inductor 16. A capacitor 22connected between ground and the relay coil 15 provides a radiofrequency ground for the tuned circuit. As shown, the cathode of thetriode 8 is connected to ground by means of a resistor 23.

In operation, the feedback necessary to sustain oscillation in theoscillator 7 is provided by means of the gridplate capacity of thetriode 8. The frequency of circuit oscillation is determined by theresonant frequency of the crystal 18. In the absence of glue, theinductor 16 is adjusted such that resonant frequency of the platecircuit is the same as the crystal frequency and the system oscillates.The presence of glue over the electrodes of the sensing element loadsand detunes the plate circuit and oscillations stop. The inductor 16 canbe adjusted to vary the amount of glue necessary to prevent oscillation.In this manner the apparatus can be made to respond not only to thecomplete absence of glue but to an inadequate amount of glue. With noglue present and the system oscillating, sufficient grid bias isdeveloped to reduce the plate circuit and deenergize the relay coil 15.Thus, any failure of the vacuum tube 9 will also de-energize the relay.In this manner the system is made substantially safe-failing.

Referring now to FIG. 2, the level of oscillation and plate current areshown plotted against sensing element capacity. As the value of theresistor 23 is made larger the conductivity change necessary to providerelay operation is decreased. It has also been found, however, thatwithout the bias provided by the resistor 21 that as the conductivitychange becomes smaller, the plate current differential betweenoscillation and non-oscillation also becomes smaller. Thus, as thesensitivity of the circuitry to glue becomes higher the current changeavailable to perform the control or indicating function becomes smaller.It has been found, however that this undesirable mode of'operation canbe overcome by the inclusion of the resistor 21. 'In this respect it hasalso been found that it is desirable to maintain the value of theresistor 21 high. When this is done, the resistor 21 does not affect theconductivity differential necessary to initiate or stop operation butserves to substantially increase the current differential between theoscillatory and non-oscillatory states of the circuit. Thus, it has beenfound possible to provide both improved relay reliability and highsensitivity.

By way of illustration and not by way of limitation, the following aretypical value for the components of a practical embodiment of thepresent invention as shown in FIG. 1 which is adapted to operate therelay 15 in response to the proximity to the sensing element 5 of a gluestrip which has a resistivity of 400,000 ohms per inch:

Vacuum tube 9 6SN7.

Resistor 19 1 meg. ohms.

Resistor 21 10 meg. ohms.

Resistor 23 2200 ohms.

Crystal 13 1 megacycle crystal. Variable inductor 16 200-300 Henries(micro). Capacitor 22 1000 mmfd.

Transformer 13 Primary volts, secondary 250 volts.

Referring now to FIG. 3 there is shown a perspective View of the sensingelement 5. The sensing element 5 comprises a base 25 made of hardrubber, Bakelite, or other suitable insulating material. As shown, thetop surface of the base 25 is curved with the high point at itsmid-section. The surface presented by the sensing element 5 as a resultof the curved shape of the base 25 aids in maintaining the sensingelement in contact with or in close proximity to the material beingtested, particularly when that material is a rapidly moving web Whilecontact with the material being tested is not essential to the operationof the system, there are distance limitations, beyond which sufiicientcapacitive coupling cannot be obtained for reliable operation of thesystem. Where glue is to be applied to relatively stiff material such ascardboard, it has been found more expedient to provide the sensingelement 5 with a flat surface.

A plurality of sensing electrodes 26, 27, 28, 29, and 31 are positionedin an in-line relationship on the top surface of the base 25 so as to besubstantially flush therewith. For this purpose, printed circuittechniques have been found applicable. As shown schematically, in FIG. 1the electrodes 26, 28 and 31 are connected together and to ground bymeans of the shielded cable 6 and the electrodes 27 and 28 are connectedto the plate of the triode 8 by means of the cable 6. The top surface ofthe base 25 A and hence the electrodes 26 through 31 are covered by asheet of insulating material 32 which is folded over the ends of thebase 25 and gripped by means of the clamping members 33 and 34. Theseclamping members are adapted to be tightened against the ends of thebase 25 by means of screws and are provided with knurled inner surfacesadapted to grip and hold the insulating film 32 against the base 25.

The insulating film 32 provides the sensing element 5 with a replaceablewipable, surface adapted to protect the electrodes from wear due to theabrasive action of material being drawn across the surface of theelement and from contamination with glue. The insulating film 32 alsoserves as a spacing member making the sensing element 5 substantiallyinsensitive to the moisture content to the material to which the glue isapplied since it is in any fringe electrostatic field which may existbetween the electrodes. The insulating film 32 is preferably athermoplastic resin such as tetrafluoroethylene, polyethylene or nylon,which are by nature highly hydrophobic and l'lOIlPOl'OUS.

As hereinbefore stated, the glue detection system of the presentinvention does not depend on the fringe effect existing at the gapbetween the adjacent electrodes to provide a detectable change incapacity. The present invention relies upon the establishment of acoupling between the electrodes and the glue, and thus the glue of othermaterial being detected must be conductive. In operation, each of theelectrodes 26, 27, 28, 29, and 31 forms a capacitor with the glue strip,and the capacitors thus formed are in turn connected in series with eachother by means of the glue strip. In this respect the electrodes aremade wide enough to provide coupling within the limits of toleratedlateral shifting of either the paper or the glue strip. While twoenergized electrodes have been shown alternately spaced between twogrounded electrodes, it should be understood that the sensing elementcan comprise but a single pair of electrodes, one energized and onegrounded; By increasing the length of the electrodes employed, it ispossible to increase the sensitivity of the system up to a certainpoint, beyond which no significant increase in sensitivity is achieved.This is due to the resistance added to circuit by the increase in thepath of conduction through the glue strip.

While for the purpose of this disclosure the present invention has beendescribed in connection with an application in which a strip of paper ismoved continuously pasta sensing element, it should be understood thatwith appropriate modifications, the present invention is equally adaptedfor applications in which glued articles move intermittently past asensing element. It should also be understood that the present inventionis adapted to detect the presence of glue or other conductive films onmaterials other than paper. For example, the present invention isapplicable for the detection of glue on such materials such ascardboard, woven fabrics, and the like.

in addition to functioning as a glue detecting apparatus, the circuit ofthe present invention can be utilized to indicate the amount of glueapplied to a given surface. When so utilized a DC. milliammeter may beconnected in series with the relay coil 15 to indicate the magnitude ofplate current flowing, and hence the amount of glue. Still further, thecircuit of the present invention may be adapted to detect the locationof glue applied to an article. For such an application, a plurality ofdetecting circuits and sensing elements may be utilized with the sensingelements positioned so as to detect the presence of glue on given areasof the material being tested. As for example, two sensing elements maybe positioned on either sides of an intended glue path to detect theplacing of glue on areas other than the intended path.

Having described the present invention, that which is claimed as new is:

1. An apparatus for detecting the presence or absence of a conductiveglue on a portion of a moving strip of paper comprising, in combination,a sensing element adapted to be positioned adjacent to said paper on thereverse side of said paper from said glued portion and opposite to theglued portion, said sensing element comprising at least a pair ofelectrodes spaced from each other, said electrodes having dimensionslarge compared with their spacing, each of said electrodes being adaptedto form a capacitor with said glued portion, the capacitors thus formedbeing connected to each other through said glue, oscillator meanscoupled to said electrodes and relay means connected to said oscillatorresponsive to the series impedance of said capacitors.

2. An apparatus for indicating the presence or absence of a conductiveglue on a portion of a moving strip of paper comprising, in combination,a sensing element adapted to be positioned adjacent to said paper on thereverse side of said paper from said glued portion and opposite to theglued portion, said sensing element comprising at least a pair ofelectrodes spaced from each other in an in-line relationship, saidelectrodes having dimensions large compared with their spacing, each ofsaid electrodes being adapted to provide a capacitive coupling with saidglued portion, said sensing element being connected to control thetuning of the plate circuit of a vacuum tube oscillator and indicatingmeans connected to said oscillator, the presence or absence of glue onthe portion of said paper opposite to said electrodes controlling theoscillation or non-oscillation of the oscillator.

3. An apparatus for indicating the presence or absence of glue on amoving strip of paper comprising, in combination, a sensing elementadapted to be positioned adjacent to the path of movement of said paperopposite the area of said paper upon which said glue is applied, saidsensing element comprising at least a pair of electrodes spaced fromeach other in an in-line relationship in line with said path of papermovement, said electrodes having dimensions large compared with theirspacing, one of said electrodes being connected to ground, oscillatormeans connected between ground and the other of said electrodes tosupply to said electrode oscillatory energy of a radio frequency, andrelay means connected to said oscillator, said electrodes formingcapacitors with said glue strip, said glue strip connecting saidcapacitors in series to control the oscillation or non-oscillation ofsaid oscillator means.

4. An apparatus for indicating the presence or absence of glue on amoving strip of paper comprising, in combination, a sensing elementadapted to be positioned adjacent to the path of movement of said paperon the side of said paper opposite to the side upon which said glue isapplied and parallel with the area of said paper upon which said glue isapplied, said sensing element presenting a surface over which saidmoving strip of paper can pass, said sensing element comprising at leasta pair of capacitor electrodes spaced from each other in an end to endrelationship in line with the path of movement of said paper, saidelectrodes having dimensions large compared with their spacing, aninsulating film covering said electrodes, one of said electrodes beingconnected to ground, oscillator means connected between ground and theother of said electrodes, said oscillator means being adapted tooscillate when glue on said paper bridges said electrodes, andindicating means connected to said oscillator, the amount of glue onstrip controlling the level of oscillation of said oscillator means.

5. An apparatus for indicating the presence or absence of a strip ofconductive glue on a moving sheet of paper comprising, in combination, asensing element adapted to be positioned adjacent to said paper on thereverse side of said paper from said glue strip and opposite to saidglued strip, said sensing element comprising a plurality of electrodesspaced from each other in an in-line relationship, said electrodeshaving dimensions large compared with their spacing, each of saidelectrodes being adapted to form a capacitor with said glue strip, thecapacitors thus formed being connected to each other through said glue,oscillator means coupled to ground and alternate ones of said electrodesto energize said alternate electrodes, the others of said electrodesbeing connected to ground and indicating means connected to saidoscillator, the presence or absence of glue on said paper controllingthe oscillation or non-oscillation of said oscillator means.

6. Apparatus as specified in claim 5 wherein said oscillator meanscomprises a tuned-plate, tuned-grid crystal controlled oscillator andsaid sensing element is connected to control the tuning of the platecircuit in response to the presence or absence of glue on said movingsheet.

References Cited in the file of this patent UNITED STATES PATENTS1,984,166 Walter Dec. 11, 1934 2,222,221 Burford Nov. 19, 1940 2,285,152Firestone June 2, 1942 2,544,673 Haber Mar. 13, 1951 2,555,977 KlineJune 2, 1951

